The present invention relates to tool steel for hot working and cold working, and more particularly to highly isotropic tool steel having high toughness and ductility, which can enjoy a long life without cracking under severe thermal and mechanical stress when used for various hot working dies such as hot forging dies, aluminum die-casting dies and aluminum extrusion dies, and whose hardness can be increased because of a high resistance to cracking so that its life in terms of wear resistance can be elongated.
Dies for cold working and hot working have recently been getting bigger, more complicated in shape, more efficient in forging due to more rapid cooling from die surfaces and more sharpened at die edges and corners for enhancing forging precision, resulting in early or premature fracture or breaking of the dies. And because higher forging precision is required, the slightest dulling and wear of die edges and surfaces has increasingly led to forged products failing to meet the required size and shape precision. Thus, the dies cannot be used for a long period of time. To prevent early or premature deformation and wear, attempts have been made to enhance the hardness of dies, but enhancing the hardness merely led to early breaking of the dies.
Tool steel is usually forged so that it is elongated in the forging direction. This direction of elongation by forging is usually called "longitudinal direction," and a direction perpendicular thereto is called "transverse direction."
In conventional tool steel for hot working, toughness is lower in the transverse direction than in the longitudinal direction. The transverse direction and the longitudinal direction are simply called T-direction and L-direction, respectively. For conventional tool steel, a ratio of T-direction toughness to L-direction toughness is usually 0.6 or so. Incidentally, the T-direction toughness means toughness measured with respect to cracks generated and propagating along fiber flows in a die during hot working, and the L-direction toughness means toughness measured with respect to cracks propagating perpendicularly to the fiber flows.
Therefore, the conventional tool steel is vulnerable to cracking and fracture along the fiber flows, so that dies made from such tool steel have a life to cracking mostly determined by the toughness and ductility thereof in the transverse direction which are lower than those in the longitudinal direction. The reason therefor is that long clusters of non-metallic inclusions extending along the direction of elongation by forging tend to be spots from which peeling and breaking initiate, so that cracks are likely to be generated and propagate along the clusters, namely fibers. Further, when there is extreme compositional segregation in the banded segregation having a large band width and extending along the direction of elongation by forging, and when it is highly aligned to the direction of fibers, cracks are likely to propagate linearly along the banded segregation.